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Buried or above the insulation

I live on the gulf coast where humidity is high in the summer and I probably spend more time cooling my house during the year than heating it.

I had an insulator give me a quote on cellulose insulation in the attic and he recommended building a U shaped box for the AC duct and burying the duct in the insulation for extra insulation of the duct. When I got a quote on the AC, the AC guys said that it was a no no and the duct should run above the insulation. Who is right?

I have been kind of gung ho on insulating my house, so the recommendation was given with the idea of doing a little extra to increase the efficiency of your cooling. The guys that told me to run the duct work above the insulation stated concerns of condensation getting in the insulation.

I have been kind of gung ho on insulating my house, so the recommendation was given with the idea of doing a little extra to increase the efficiency of your cooling. The guys that told me to run the duct work above the insulation stated concerns of condensation getting in the insulation.

He would be correct, what ever insulation that is covering the duct would need to have a vapor barrier over it to prevent condensation from being formed in the insulation.

I have been kind of gung ho on insulating my house, so the recommendation was given with the idea of doing a little extra to increase the efficiency of your cooling. The guys that told me to run the duct work above the insulation stated concerns of condensation getting in the insulation.

Without a vapour barrier, condensation may be a concern, I have seen the insulation blown over ductwork without an issue, but I think the best solution would be to properly insulate the duct work separately and tape with something that would keep the moisture at bay. Can't think of the name, but that fibreglass rigid insulation with the silver backing would be best, than seal seams with foil tape.

in the hot humid south we don't bury ducts in insulation
or build boxes filled with insulation to put ducts inside.

even with ducts lying on insulation on attic floor, the ducts
condensate where they touch the insulation.
surrounding the duct with insulation causes condensation to
surround the duct.

there are lots of better uses for the money you'd spend to
build boxes around ducts & put insulation inside.

strapping ducts off attic floor with a 3" duct strap so that
there are no bends or restrictions.
mastic sealing ducts to plenum, mastic sealing plenums to equipment,
sealing returns air tight & sealing supply boxes to attic floor/ceiling of living
space.

these are investments that work for us.
you'll have better air flow with ducts strapped properly.
more air entering the house instead of leaking into attic with
mastic seal.
less dirt buildup on supply grills with the gap hidden behind the
supply grill mastic sealed.
use either paint on mastics on ductwork & mastic tape for
supply boxes.

do a little experiement yourself.
put one of the ducts on the insulation on the attic floor
on a hot day. the next morning go into the attic, pick the
duct off the insulation..you'll see/feel condensation.
dew point happens at different times in a 24 hour period..
so check periodically during the day to see when condensation
has dried.

usually it is from people in climates other than hot humid that
recommend putting insulation over ductwork.

think about who recommended the insulation...the guy selling
insulation.
the hvac guy knows that the vapor barrier on the duct will reach
dewpoint & condensation will form, wetting insulation.

sometimes insulators should just insulate & not speculate on
things they don't understand.

and btw...with cellulose the fine particles will enter the living space
through every small gap & crack in the ceiling.
at minimum you should seal supply boxes with mastic tape.
here is a pic of how I do this from inside the house.
I use only hardcast brand #1402 mastic tape
as it will seal from any type of ceiling into the metal
lip of supply box.

best of luck.

The cure of the part should not be attempted without the cure of the whole. ~Plato